The present invention relates generally to a force sensor assembly for use in infusion pumps. More particularly, the present invention relates to an assembly that reduces a load cell""s sensitivity to the placement of an intravenous tube on the force sensor assembly in an infusion pump.
Various devices have been developed to administer intravenous (IV) fluids to patients. One such device, a peristaltic infusion pump, operates a series of fingers or rollers which deform and occlude a resiliently deformable IV drip tube at multiple points sequentially along the tube""s length. These occlusions form a wave like motion which forces the IV fluid under positive pressure along the tube. After each successive occlusion, the tube resiliently rebounds to its original diameter. The repetitive deformation of the tube may, however, ultimately weaken the resilience of the tube material. After prolonged use, a tube may not fully rebound to its former shape, thereby partly or fully occluding the tube at a point along its path. In addition, IV drip sets often contain clamps, which can be inadvertently be left closed, thus partly or fully occluding the tube.
In order to effectively control IV fluid delivery, it is essential that the infusion system constantly determine whether fluid is in fact being delivered to the patient. Interruptions of the fluid flow may occur for number of reasons, such as for example, occlusion of the tube or a blocked catheter. If the pump mechanism does not stop when the tube is occluded, either the pump will stall, the pump will continue to run with no fluid delivered, or the fluid pressure in the tube will increase until the obstruction catastrophically clears, possibly injuring the patient.
Accordingly, many infusion pump systems include a force or pressure sensor to determine whether there is an increase or loss of pressure within the tube. The sensor determines whether the fluid flow in the tube has been interrupted, and the pumping mechanism may be stopped and/or medical personnel notified. Because of the potentially harmful consequences of such interruptions, it is important that these sensors be as accurate and reliable as possible. Also, due to an infusion pump""s portability and arduous operating conditions, it is desirable for these sensors to be small and rugged.
Force or pressure sensors used in infusion pumps typically contain a plunger that is either constrained in some way, such as with a pin in a hole, or of a free floating type. A constrained plunger type force sensor assembly could be comprised of an actuation plunger connected to a pin positioned and guided within a hole in a sensor housing. A transducer or load cell is positioned along the central axis of the plunger, remote from an IV drip tube. When the IV drip tube is positioned directly over the central axis of the plunger, a force created by the internal pressure of the IV drip tube is applied via the plunger and pin to the load cell, which measures the applied force. In this scenario, the measured force would be comparatively accurate as there is typically only a small loss of transferred force due to friction. However, when the IV drip tube is positioned off-center to the plunger""s central axis, the plunger tends to rotate causing side loading on the plunger pin by the sensor housing which may bind the plunger pin in the hole. This side loading creates a friction force between the plunger pin and the sensor housing which results in a loss of force being applied to the load cell. This friction force ultimately leads to inaccurate and unreliable force measurement results. The above scenario may occur for example where an IV tube is misplaced on the force sensor assembly or where the tube drifts along the plunger surface during use.
A free floating plunger type force sensor could be comprised of an actuation plunger positioned within a hole in a sensor housing providing clearance allowing the plunger to float freely. The plunger is positioned over a well of force transmitting gel. The plunger transfers force applied by the IV tubing to the gel, which in turn transfers the force to a transducer or load cell situated within the gel. An example of a gel-type sensor is disclosed in U.S. Pat. No. 5,661,245. The plunger is typically allowed to angulate when a force is applied by the IV tubing to the plunger along an off-center axis. The angled plunger transfers force to the gel with less efficiency, which in turn transfers less force to the transducer or load cell situated within the gel. This reduced force ultimately leads to inaccuracies in the force measurement results. Additional inaccuracies may also be experienced due to frictional reaction forces between the plunger edges and the sensor housing.
Accordingly, there is a need for a small and inexpensive force sensor assembly which provides accurate and reliable results regardless of the placement of the IV tubing on the force sensor assembly.
According to the invention there is provided a force sensor assembly which is adapted to reduce a load cell""s sensitivity to the positioning of an IV tube on the load cell""s actuation plunger. The force sensor assembly comprises a housing, a load cell at least partially disposed within the housing and a plunger, which is pivotable about an axis. The plunger may be hinged to the housing or to another fixed point proximate to the housing, and may for example be a living hinge or a small pin pivot hinge. The plunger further comprises an upper surface which may be shaped to compensate for variations in measured force-caused by possible misalignment of the IV tubing on the plunger""s upper surface. The plunger also comprises an underside surface distal from the upper surface.
In use an IV tube is placed on the plunger""s upper surface, pressure within the IV tube applies a force to the upper surface of the plunger, causing the plunger to pivot about the axis. The pivoting plunger""s underside makes contact with the load cell and thereby wholly transfers the applied force to the sensor for measurement.